1. Field of the Invention
The present invention relates to the field of automotive rectifier assemblies. Particularly, the present invention relates to a method and apparatus for preventing rectifier assemblies from overheating.
2. Description of the Related Art
Advances in technology have allowed for a reduction in the size of automotive alternators (herein xe2x80x9calternatorsxe2x80x9d). Although alternators have become smaller, the electrical energy output requirements have increased. Generally, recharging an automobile""s battery requires a current between 40 and 50 amperes. Combined with the energy requirements of the air conditioning system, the computer module, the car radio, the fans, and the lighting systems, the overall current consumption can exceed 150 amperes.
The high current alternator is generally not able to dissipate heat out of the rectifier module fast enough to prevent semiconductor failure. The problem is particularly severe during the summer months, when the ambient temperature is quite high, thus reducing the rate of heat transfer between the rectifier module and the surrounding environment.
Polyphase alternating current can be converted to direct current suitable for use in an automotive electrical system by conducting current through semiconductor diodes in a rectifier circuit. The semiconductors may be affixed directly onto a heat sink, as is illustrated by U.S. Pat. No. 5,005,069, or press-fit into pre-punched holes in the heat sinks, as is illustrated by U.S. Pat. No. 5,043,614. In other methods, such as that illustrated in U.S. Pat. No. 4,799,309, the semiconductors are affixed onto integrated heat sinks. The heat control methods identified above are usually difficult to implement because the semiconductors are extremely sensitive to heat, stress, and mechanical force applied to the semiconductors during the manufacturing and installation. The stress can cause premature semiconductor failure during vehicle operation.
The likelihood of failure is especially great when the semiconductors of the rectifier assembly are affixed onto a single, integrated, aluminum heat sink. The semiconductors are usually encapsulated with heat conductive epoxy, which prevents the semiconductors from expanding or from dissipating heat efficiently. The semiconductor overheating and failure conditions has been historically demonstrated by the FORD IAR alternator catastrophic failure scenario. Therefore, there is a need for a method of ensuring that the rectifier assembly does not overheat when semiconductors fail while not overstressing the semiconductors during assembly.
Automotive power requirements utilizing a rectifier can exceed 70 amperers. With the present day high under-hood temperatures, along with the heat generated by the alternator and the rectifier, this high current cannot safely pass through the rectifier male terminal blades and into the female connector terminals when the terminals are not properly mated.
Most rectifier assemblies use three male terminal blades molded into a connector housing. The B+blades that supply the battery power are formed out of tin plated brass or steel and are bent into a xe2x80x9cUxe2x80x9d configuration (usually a square bend molded into a housing, and having no flexibility) to carry the high current. The third independent blade is used to transfer low amperage stator alternating current to the electric choke circuit.
In the prior art, when the original alternator, rectifier and connector are manufactured, assembled and installed by the manufacturer, the system operates quite well for several years. However, after operating for several years, under the stress of high current and high under-hood temperatures, the materials take on a preset form, or memory.
Replacing a failed alternator presents a major problem for the re-manufacturer and the installer because the installer must force and pry off the tightly fit female mating connector. After installing a remanufactured alternator, the mating connector is mechanically distorted, thermally aged, or has a preset memory. Thus, the connector terminal blades most likely will not align with the female receptacle terminals, creating a high resistance loose connection, causing arcing, over-heating, and introducing a fire hazard.
In an attempt to solve the problem, many large volume alternator re-manufacturers enclose a new connector plug with every alternator sold. This practice is extremely expensive, and cannot guarantee the rectifier contact blades will be properly aligned to provide a low resistance tightly fit connection after the installer forces the new connector into the re-manufactured alternator rectifier.
Other alternator re-manufacturers recommend that their customers perform a 6 pound pull test on the connector plug prior to plugging it into the newly installed alternator. A 6 pound weight is attached to a single male terminal blade. The blade is then plugged into each of the three female receptacles. If the weight causes the male blade to pull out of any one of the three female receptacles, the existing automobile""s connector must be cut out and a new connector is spliced into the circuit. The installer must then force the new female connector from side to side, while pushing it downward into the male housing, allowing the male blades to enter into the female receptacles. This action causes the male blades to bend.
Because the male terminal blades cannot self-align, they lose their required contact surface area, and create a high resistance connection. This connection becomes a hot spot within the connector housing because of the high operating current conducted through it. The extra heat generated within the re-manufactured alternator will not allow it to dissipate out of the rectifier. As heat continues to build up within the rectifier it either fails or becomes a fire hazard.
In accordance with the present invention, a rectifier assembly employs semiconductor circuits that automatically open whenever the semiconductors fail and dissipate a predetermined level of heat.
In one embodiment, the present invention utilizes spring-loaded terminals to connect the semiconductor circuits such that, when a failure occurs, the high temperature causes a preselected soldered joint to melt. Once melted, a compressed spring, under the joint, holds the terminals away from one another to open the failed circuit and stop the current flow.
In one embodiment, the rectifier assembly includes six spring-loaded diodes affixed onto two copper heat sinks. The heat sinks provide cooler and more efficient operation as described in U.S. Pat. No. 5,659,212, incorporated herein by reference.
The present invention is concerned with the high power and under-hood temperatures required by modern day automotive electronics and the catastrophic fire and melt down hazards caused by overheated semiconductors. The method of the present invention avoids overstressing the semiconductors by preventing the circuits from operating in a range of operation that is beyond the semiconductor""s handling specifications. The thermal protection of the present invention virtually eliminates the automobile""s catastrophic fire hazard, the dead battery nuisance conditions, and other conditions that are associated with semiconductor failures.
The present invention also offers a method for assembling rectifiers without overstressing the diodes in the process of securing the diodes to the rectifier assembly. The method includes providing a protective cup that is used to hold the semiconductor diode and absorb stress that may otherwise be absorbed by the semiconductor body.
The rectifier of the present invention employs a terminal connector that utilizes dimpled or detented (e.g., corrugated), spring-loaded, self-aligning male terminal blades to compensate for tolerances between all manufactured connectors. The terminal blades also compensate for the existing automobile connector which may be out of tolerance, because of thermal aging, mechanical abuse, or preset memory.